This is already possible (although currently undocumented) in the standalone version of iD: OpenStreetMap – (note the /id/ URL). With the embedded version of iD (/edit/?editor=id), the #id=… parameter is not available in order to not clash with the existing node/way/relation parameters which do indeed only accept a single object. For MapRoulette, you could perhaps simply link to the standalone version of iD, which allows you to use the full spectrum of hash parameters and has the extra benefit of using the screen space slightly more efficiently?!
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